1. Field of the Invention
The present invention pertains to tantalum and tantalum nitride films which can be stress-tuned to be in tension or in compression or to have a particularly low stress, and to a method of producing such films. These stress-tuned films are particularly useful in semiconductor interconnect structures, where they can be used to balance the stress within a stack of layers, which includes a combination of barrier layers, wetting layers, and conductive layers, for example. The low stress tantalum and tantalum nitride films are particularly suited for the lining of vias and trenches having high aspect ratios.
2. Brief Description of the Background Art
A typical process for producing a multilevel structure having feature sizes in the range of 0.5 micron (xcexcm) or less would include: blanket deposition of a dielectric material; patterning of the dielectric material to form openings; deposition of a diffusion barrier layer and, optionally, a wetting layer to line the openings; deposition of a conductive material onto the substrate in sufficient thickness to fill the openings; and removal of excessive conductive material from the substrate surface using chemical, mechanical, or combined chemical-mechanical polishing techniques. Future technological requirements have placed a focus on the replacement of aluminum (and aluminum alloys) by copper as the conductive material. As a result, there is an increased interest in tantalum nitride barrier layers and in tantalum barrier/wetting layers, which are preferred for use in combination with copper.
Tantalum nitride barrier films, Ta2N and TaN, have been shown to function up to 700xc2x0 C. and 750xc2x0 C., respectively, without the diffusion of copper into an underlying silicon (Si) substrate. Tantalum barrier/wetting films have been shown to function at temperatures of approximately 500xc2x0 C. It is advantageous in terms of processing simplicity to sputter the barrier and/or Wetting layers underlying the copper. Tantalum nitride barrier layers are most commonly prepared using reactive physical sputtering, typically with magnetron cathodes, where the sputtering target is tantalum and nitrogen is introduced into the reaction chamber.
S. M. Rossnagel et al. describe a technique which enables control of the degree of directionality in the deposition of diffusion barriers in their paper titled xe2x80x9cThin, high atomic weight refractory film deposition for diffusion barrier, adhesion layer, and seed layer applicationsxe2x80x9d, J. Vac. Sci. Technol. B 14(3) (May/Jun. 1996). In particular, the paper describes a method of depositing tantalum (Ta) which permits the deposition of the tantalum atoms on steep sidewalls of interconnect vias and trenches. The method uses conventional, non-collimated magnetron sputtering at low pressures, with improved directionality of the depositing atoms. The improved directionality is achieved by increasing the distance between the cathode and the workpiece surface (the throw) and by reducing the argon pressure during sputtering. For a film deposited with commercial cathodes (Applied Materials Endura(copyright) class; circular planar cathode with a diameter of 30 cm) and rotating magnet defined erosion paths, a throw distance of 25 cm is said to be approximately equal to an interposed collimator of aspect ratio near 1.0. In the present disclosure, use of this xe2x80x9clong throwxe2x80x9d technique with traditional, non-collimated magnetron sputtering at low pressures is referred to as xe2x80x9cgamma sputteringxe2x80x9d.
Gamma sputtering enables the deposition of thin, conformal coatings on sidewalls of a trench having an aspect ratio of 2.8: 1 for 0.5, xcexcm wide trench features. However, we have determined that gamma-sputtered TaN films exhibit a relatively high film residual compressive stress, in the range of about xe2x88x921.0xc3x9710+10 to about xe2x88x925.0xc3x9710+10 dynes/cm2. High film residual compressive stress, in the range described above, can cause a Ta film or a tantalum nitride (e.g., Ta2N or TaN) film to peel off from the underlying substrate (typically silicon oxide dielectric). In the alternative, the film stress can cause feature distortion on the substrate (typically a silicon wafer) surface or even deformation of a thin wafer.
A method of reducing the residual stress in a Ta barrier/wetting film or a Ta2N or TaN barrier film would be beneficial in enabling the execution of subsequent process steps without delamination of such films from trench and via sidewalls or other interconnect features. This reduces the number of particles generated, increasing device yield during production. In addition, a film having a near zero stress condition improves the reliability of the device itself.
We have discovered that the residual stress residing in a tantalum (Ta) film or a tantalum nitride (TaNx, where 0 less than xxe2x89xa61.5) film can be controlled (tuned) by controlling particular process variables during deposition of the film. Process variables of particular interest for sputter-applied Ta and TaNx films include the following: An increase in the power to the sputtering target (typically DC) increases the compressive stress component in the film. An increase in the process chamber pressure (i.e., the concentration of various gases and ions present in the chamber) increases the tensile stress component in the film. An increase in the substrate DC offset bias voltage (typically an increase in the applied AC as substrate bias power) increases the compressive stress component in the film. When the sputtering is IMP sputtering, an increase in the power to the ionization coil increases the compressive stress component in the film. The substrate temperature during deposition of the film also affects the film residual stress. Of these variables, an increase in the process chamber pressure and an increase in the substrate offset bias most significantly affect the tensile and compressive increases the compressive stress components, respectively.
The most advantageous tuning of a sputtered film is achieved using Ion Metal Plasma (IMP) sputter deposition as the film deposition method. This sputtering method provides for particular control over the ion bombardment of the depositing film surface. When it is desired to produce a film having minimal residual stress, particular care must be taken to control the amount of ion bombardment of the depositing film surface, as an excess of such ion bombardment can result in an increase in the residual compressive stress component in the deposited film.
Tantalum (Ta) films deposited using the IMP sputter deposition method typically exhibit a residual stress ranging from about +1xc3x971+10 dynes/cm2 (tensile stress) to about xe2x88x922xc3x9710xe2x88x9210 dynes/cm2 (compressive stress), depending on the process variables described above. Tantalum nitride (TaNx) films deposited using the IMP method typically can be tuned to exhibit a residual stress within the same range as that specified above with respect to Ta films. We have been able to reduce the residual stress in either the Ta or TaNx films to low values ranging from about +1xc3x9710+9 dynes/cm2 to about xe2x88x922xc3x9710+9 dynes/cm2 using tuning techniques described herein. These film residual stress values are significantly less than those observed for traditionally sputtered films and for gamma-sputtered films. This reduction in film residual compressive stress is particularly attributed to bombardment of the film surface by IMP-generated ions during. the film deposition process. Heavy bombardment of the film surface by IMP-generated ions can increase the film residual compressive stress, so when it is desired to minimize the film compressive stress, the ion bombardment should be optimized for this purpose.
Other process variables which may be used in tuning the film stress include the spacing between the sputter target and the substrate surface to be sputter deposited; ion bombardment subsequent to film deposition; and annealing of the film during or after deposition.
We have also discovered that Ta and TaNx films deposited using physical vapor deposition techniques over a particular temperature range have improved diffusion barrier properties. The lower end of the temperature range is approximately 300xc2x0 C. Using a deposition temperature of 300xc2x0 C. or higher, the barrier layer can be thinner, leading to better manufacturing through-put. Further, a thinner barrier layer requires less chemical-mechanical polishing to remove residual barrier layer from a substrate surface surrounding a copper interconnect, and this reduces the amount of copper which is removed from the interconnect due to the polishing action (copper xe2x80x9cdishingxe2x80x9d). Accordingly, disclosed herein is an improved method of depositing a Ta or TaNx barrier layer for use in combination with copper in a semiconductor interconnect structure. The improvement comprises depositing a Ta or TaNx (where 0 less than xxe2x89xa61.5) barrier layer at a substrate temperature of at least 300xc2x0 C., preferably, within a temperature range of about 300xc2x0 C. to about 600xc2x0 C., most preferably, within the range of about 300xc2x0 C. to about 500xc2x0 C., prior to deposition of copper on the substrate.